const addStrings = (num1, num2) => {
  while (num1.length < num2.length) num1 = "0" + num1;
  while (num1.length > num2.length) num2 = "0" + num2;
  let carry = 0;
  let res = "";
  for (let i = num1.length - 1; i >= 0; i--) {
    const sum = +num1[i] + +num2[i] + carry;
    res = (sum % 10) + res;
    carry = sum >= 10 ? 1 : 0;
  }
  return carry === 1 ? "1" + res : res;
};
